1. Field of the Invention
The present invention relates to a drive method and drive apparatus for an ink-on-demand type ink jet head, and particularly to a drive method and drive apparatus for eliminating the effects of residual charges in the diaphragm of an electrostatic ink jet head actuator.
2. Description of the Related Art Ink jet recording apparatuses offer numerous benefits, including extremely quiet operation when recording, high speed printing, a high degree of freedom in ink selection, and the ability to use low-cost plain paper. The so-called "ink-on-demand" drive method whereby ink is output only when required for recording is now the mainstream in such recording apparatuses because it is not necessary to recover ink not needed for recording.
The ink jet heads used in this ink-on-demand method commonly use a piezoelectric device for the drive means as described in JP-B-1990-51734, or ejection of the ink by means of pressure generated by heating the ink to generate bubbles as described in JP-B-1986-59911.
Japanese Patent Laid-open No. 1990-24218 also describes a drive method having a piezoelectric device. This drive method comprises a piezoelectric device for varying the volume of the pressure chamber generating the ink eject pressure. During the printer standby state, an electrical pulse is applied to the piezoelectric device in the same direction as the polarization voltage of the piezoelectric device, thereby charging the piezoelectric device and reducing the volume of the pressure chamber. To eject the ink during printing, the piezoelectric device is gradually discharged to increase the volume of the pressure chamber, and an electrical pulse is again applied to the piezoelectric device to rapidly charge the device and decrease the pressure chamber volume, thereby ejecting ink from the nozzle. To eject the ink with greatest efficiency at a low voltage level, a voltage is again applied to the piezoelectric device to rapidly decrease the pressure chamber volume near the peak value of the damped vibration of the ink supply system occurring when ink is suctioned into the pressure chamber.
The following problems, however, are presented by these conventional ink jet heads.
In the former method using a piezoelectric device, the process of bonding the piezoelectric chip to the diaphragms used to produce pressure in the pressure chamber is complex. With current ink jet recording apparatuses having plural nozzles and a high nozzle density to meet the demand for high speed, high quality printing, these piezoelectric devices must be precisely manufactured and bonded to the diaphragms, processes that are extremely complicated and time-consuming. As the nozzle density has increased, it has become necessary to process the piezoelectric devices having a width in the order of magnitude of several ten to hundred microns. With the dimensional and shape precision achievable using current machining processes, however, it is difficult to manufacture with precision such devices. Accordingly, there is a wide variation in print quality.
In the latter method whereby the ink is heated, the drive means is a thin-film resistive heater that generally eliminates the above problems. However, this type of device has other problems. For example, the resistive heater has a tendency to become damaged over time, and the practical service life of the ink jet head is accordingly short. This is believed to be caused by the repeated rapid, heating and cooling of the drive means and the impact of bubble dissipation.
An ink jet head using an electrostatic actuator is described in U.S. Pat. No. 4,520,375. This type of ink jet head is provided by a pair of spaced capacitor plates, one of which is a thin diaphragm, preferably of semiconductor material, such as silicon, and a reservoir containing a fluid, such as ink. The diaphragm communicates with a nozzle. Impressing a time varying voltage on the capacitor causes the diaphragm to be set into mechanical motion, and the fluid to exit through the nozzle responsive to the diaphragm motion.
However, the drive apparatus or method that efficiently utilizes the characteristics of the semiconductor substrate to drive the ink jet head employing an electrostatic force has not been described in detail. In these conventional devices, it has not been possible to assure more stable drive characteristics.
One problem is that there may be a large difference in the current value according to the polarity of the applied voltage in the contact of the metal and semiconductor in the electrode because of the affect of the space-charge layer (also known as "depletion layer").
The space-charge layer is regarded as a capacitor not a conductor, and causes undesirable phenomena for an actuator of an ink jet head, for example, a decrease in displacement of the diaphragm, or an increase of the drive voltage to eject the ink droplets.
Regarding this problem, in U.S. Pat. No. 4,520,375, a time varying voltage is impressed on the capacitor which causes the diaphragm to be set into mechanical motion and the fluid to exit responsive to the diaphragm motion. However, U.S. Pat. No. 4,520,375 provides little guidance about the characteristics of semiconductor materials or few details on how to effectively drive such a print head.
In the case of the capacitor plate having the diaphragm is P-type semiconductor substrate and an alternating voltage having no bias voltage is applied to the actuator, the substrate acts as a conductor when a positive charge is applied to the substrate electrode, but when a negative charge is applied, the substrate does not act as a conductor and has capacitance due to the presence of the space-charge layer. As a result, the displacement of the diaphragm having applied a positive voltage is different from that having applied a negative voltage. As a result of this condition, there is a tendency of the ink droplets not being ejected uniformly, which deteriorates a print quality.
In another example, an alternating voltage is added to a bias voltage so that the polarity of voltage applied to the diaphragm is fixed. In this situation, a very large voltage is needed to deform the diaphragm and eject ink due to the presence of the space-charge layer if the applied voltage has an unsuitable polarity.
The following is a detailed description of the operation principal of an electrostatic actuator for applying to ink jet head.
When a voltage is applied to the gap between the diaphragm and an oppositely placed electrode, the resulting electrostatic force causes the electrode to attract the diaphragm, thus bending it. On the other hand, when bent, the diaphragm generates a restoring force in the opposite direction. Therefore, the extent of the bending of the diaphragm during the application of a voltage to the electrostatic actuator, i.e., the displacement of the mid-section of the diaphragm (hereinafter referred to as "the extent of the diaphragm displacement" or "diaphragm displacement") represents a value at which the electrostatic force and the diaphragm's restoring force are in equilibrium. If P denotes the restoring force of the diaphragm, x the displacement, and C the compliance of the diaphragm, the three variables can be expressed in the following equation: EQU P=x/C (1)
Likewise, if Va denotes the effective voltage, G the distance between the diaphragm and the electrode (hereinafar "electric gap length"), and e the permittivity of the gap, then the electrostatic force generated between the diaphragm and the electrode can be expressed as: EQU P=e/2{Va/(G-X)}.sup.2 ( 2)
The position at which the displacement of the diaphragm comes into equilibrium can be determined from Equations (1) and (2).
FIG. 26 is a characteristic chart depicting the relationship between the displacement and the restoring force of the diaphragm and the relationship between the displacement of the diaphragm and the electrostatic force that is generated. These relationships are obtained from Equations (1) and (2), respectively. In the figure, diaphragm displacement x is plotted on the horizontal axis, and the pressure generated by the restoring force of the diaphragm and the pressure generated by the electrostatic force are plotted on the vertical axis. The following parameters, used in the experiment, are also used in the calculations: EQU C=5.times.10.sup.-18 [m.sup.5 /N], G=0.25 [.mu.m], e=8.85 [pF/m]
The electrostatic forces, calculated for each applied voltage, are shown by curves in the figure. The relationship between the diaphragm displacement and the diaphragm restoring force is indicated by a straight line. Of two intersections between the straight line and each curve, the intersection on the left side indicates the extent of bending (displacement quantity) of the diaphragm at the particular voltage level that is applied. At a voltage level at which the restoring force and the electrostatic force of the diaphragm do not intersect (e.g., 35 V), the electrostatic force is always greater than the restoring force of the diaphragm, irrespective of the displacement of the diaphragm. Therefore, in this case the displacement tends toward infinity. In actuality, however, the existence of an oppositely placed electrode limits the displacement of the diaphragm to the position of the electrode. In applying such electrostatic actuators as described above to ink jet heads for actual printer products, there remain some problems to be solved as described below.
Improving the printing speed of a printer requires an increase in the frequency in which the ink jet head pumps out ink continuously, i.e., the response frequency of the ink jet head. When attempting to achieve a high response rate for the diaphragm, if the volume of the ink ejection chamber is increased rapidly by applying sudden pulse voltages and by supplying an electrical charge between the diaphragm and the electrode, in order to attract the diaphragm to the electrode rapidly, air bubbles intrude into the ink ejection chamber from the nozzle connected to the ink channel. In other words, the rapid vibrations of the ink in the ink ejection chamber cause the gases dissolved therein, such as the nitrogen, to bubble up. As a result of these bubbles in the ink ejection chamber, any increase in pressure due to the decrease in volume of the ink ejection chamber caused by the sudden discharge of the electrical charge accumulated between the diaphragm and the electrode is absorbed or attenuated by the bubbles, thus preventing effective ink ejection. Further, the rapid attraction of the diaphragm to the electrode causes secondary vibrations of the diaphragm which often causes the violent collision of the diaphragm against opposing electrode resulting in damage to the ink jet head.
In addition to the above problem, electrostatic actuators tend to be driven improperly by external noise and induction noise because they can be driven by a few electrical charge. In particular, since the electrostatic actuators of the on-demand type printers are often driven separately from their neighboring electrostatic actuators, the neighboring electrostatic actuators sometimes operate improperly due to the induction noise generated by the driving current for the electrostatic actuator disposed side by side. Also in the operation of this kind of printers, the driving interval, namely the period between one ink ejection and the next ink ejection, often becomes fairly long. In such cases, the problem of malfunction caused by external noise arises.
The inventors have observed conventional ink jet head drive method is a very viable method for driving ink jet heads using a piezoelectric device as the actuator. However, when a piezoelectric device drive method as described above is simply applied in the ink jet head using an electrostatic actuator as shown U.S. Pat. No. 4,520,375, however, the following problems make a practical ink-on-demand type device hard to achieve.
The inventors have found that a residual charge remains in the dielectric body between the diaphragm and electrode after a pulse voltage is applied between the diaphragm and individual electrodes in ink jet heads using the electrostatic actuator. The field generated by this residual charge decreases the relative displacement of the diaphragm and individual electrodes.
This decrement in the relative displacement is a cause of insufficient ink ejection volume and reduced printing speed, which tends to lead to low print quality. This is evident in character density and pixel shifting, and in lower reliability as evidenced by dropped pixels.
In addition, the magnitude of this residual charge tends to vary due to the hysteresis of past applied voltages. As a result, the relative displacement of the diaphragm and individual electrodes is indefinite and unstable, causing further instability in the ink ejection volume and ejection speed. These factors further contributing to low print quality evident in character density and pixel shifting, and in lower reliability as evidenced by dropped pixels.
These are peculiar problems to the static electricity actuator and piezoelectric device-type heads don't have the mentioned problems.